Articulated vehicles, such as wheeled feller bunchers, skidders, forwarders, front end loaders and many other industrial vehicles, have a chassis consisting of two or more frames hinged together so that steering is effected by varying the angle of articulation between the frames.
The required articulation is accomplished using hydraulic cylinders that are connected to the frames. The hydraulic cylinders are typically controlled by a directional control valve which is used to supply hydraulic fluid to the cylinders to change the cylinder length, thereby varying the articulation angle between the frames. Hydraulic fluid is only supplied to change the cylinder length. Otherwise the cylinders act as a solid link, and prevent the frames from articulating to some other position. To permit the cylinder to function as a solid link, four way, three position control valves having closed-center ports are used.
However, typical articulated vehicles have various inherent problems. One problem is that they have only a single steering sensitivity level. Because regular directional control valves are used, hydraulic fluid is either supplied to the hydraulic cylinders or it is not. This only provides a single steering sensitivity for the vehicle. However, different steering sensitivity levels are required depending on the operation being performed. For example, the steering sensitivity required while harvesting (in low gear) are quite different from those required while traveling by road (in high gear).
A second problem is that typical articulated vehicles require constant steering throughout an operation. For example, if a typical articulated vehicle is turned to the right, the operator must then steer the vehicle back to the left to return to traveling in a straight path. Typical articulated vehicles will not automatically return to traveling in a straight path when the steering control is operated “hands-off”, unlike typical passenger cars that will. This makes the vehicles more difficult to control and more fatiguing for an operator since they must constantly be steering the vehicle.
A third problem is that typical articulated vehicles can have different size tires installed and in some cases, when larger tires are installed, the larger tires interfere with steering by contacting each other at a certain amount of articulation. To prevent this problem, a stop must be installed to prevent articulation of the frames to a point where the tires will make contact. Since this stopping position inherently involves less than a full cylinder stroke, hydraulic cushions are not feasible. In addition, typical solid stops result in a violent action and a very abrupt stop.
A fourth problem is that typical articulated vehicles must have a separate design for each steering device desired (i.e. steering wheel v. joystick). Currently, building one version or the other involves many parts special to the selected steering system. For instance, if a steering wheel is selected, then a rotary directional control valve (either Orbitrol or Quick steer) is connected to the steering wheel and the hydraulic lines used are unique to that system. Conversely, if a joystick is selected a special directional control valve must be installed and an entirely different set of conduits is required.
It would therefore be advantageous if a steering system for an articulated vehicle could be designed that: 1) allowed for various steering sensitivity levels; 2) did not require continual steering by an operator throughout an operation; 3) had a more controlled and less violent stopping mechanism at maximum articulation; and 4) did not require a separate design for each available steering device. In particular, it would be advantageous if the steering system were designed to provide different steering sensitivity levels depending on the gear that the vehicle is in, or depending on operator selection, to provide a self centering action when no steering input is supplied, to provide a controlled, soft stopping action when the vehicle reaches maximum articulation, and to allow for a single design that would accept any desired steering device without modifying or redesigning other portions of the vehicle.